Rovers, cranes, robots and other equipment for in-situ resource utilization, site scouting and surface robotic exploration on lunar surface, moons and asteroids may require position encoders that operate in extreme environments, accommodate a broad temperature range, and tolerate hard vacuum and abrasive dust. Overlapping requirements, including vacuum compatibility and reliable operation in harsh environments, may be seen in demanding industrial applications, such as in the manufacturing processes for semiconductor devices.
Optical encoders are one of the most popular conventional solutions for precision motion control applications. A typical conventional rotary optical encoder includes a glass or plastic disk with transparent and opaque areas, a light source and a photo detector array (read-head) that detects the optical pattern resulting from the position of the disk. Recent technical developments have focused primarily on performance improvements, miniaturization, efficient coding patterns and cost reduction. See, e.g., U.S. Pub. No. 2010/0213358, incorporated by reference herein. Due to their principle of operation and a relatively small size of the features on the disk, conventional optical encoders may be sensitive to contamination, e.g., dust, dirt, and the like. Although vacuum compatible solutions may exist, the read-head and, in some applications, the attachment method of the disk to the shaft represent out-gassing problems and may lead to considerable limitations in environments with aggressive residual gases.
Conventional magnetic encoders, which typically include a magnetic wheel and a magnetoresistive or Hall-effect sensor module, may not be sensitive to dust and dirt and may offer a rugged alternative to optical encoders in harsh environments. Recent research has been focused on improved accuracy and resolution while preserving the ruggedness of the magnetic solution. See e.g., Lequesne et al., “High-Accuracy Magnetic Position Encoder Concept”, IEEE Transactions on Industry Applications, Vol. 35, No. 3, May/June 1999, pp. 568-576, incorporated by reference herein. Conventional magnetic encodes typically provide a limited measurement resolution and may be sensitive to temperature effects.
Conventional capacitive linear encoders typically function by sensing the capacitance between a reader and scale. Some of the most recent research efforts have been focused on linear flexible solutions with a non tethered slider where electrostatic induction is utilized to eliminate cabling to the moving part of the system. See e.g., U.S. Pat. No. 7,199,727, and Kimura et al., “Capacitive-Type Flexible Linear Encoder with Untethered Slider Using Electrostatic Induction”, IEEE Sensors Journal, Vol. 10, No. 5, May 2010, pp. 972-978, both incorporated by reference herein. Capacitive encoders may be insensitive to external magnetic fields but may be affected by temperature, humidity and condensation, and foreign matter.
Conventional inductive encoders rely on principle that the inductance of one or more coils changes in relation to the material sensed, e.g., a semi-circular iron core. The drawbacks to inductive encoders may include the temperature dependence of the soft iron. Therefore, highly accurate inductive encoders do not include iron and the contrast is generated with eddy currents. See e.g., U.S. Pat. No. 3,820,110, incorporated by reference herein.
All of the conventional encoder technologies discussed above utilize cyclical patterns on a moving section of the system and may provide an incremental position measurement based on the number of cycles counted. In order to determine the absolute position of the moving section of the system, either at start-up or periodically/on-demand during operation, additional information needs to be coded and sensed.
In a majority of commercial products, multiple tracks on a complex disk or scale are utilized to hold information for absolute position detection with each track providing the state of one bit of a digital word that represents the corresponding absolute location. Typically, a Gray-type code is utilized to ensure that only single-bit transitions occur as the disk or scale moves. See e.g., U.S. Pat. No. 2,632,058, incorporated by reference herein.
It has been shown that a single measurement track can be utilized for absolute position detection. See e.g., Yan et al., “Coding of Shared Track Gray Encoder”, Journal of Dynamic Systems, Measurement, and Control, Vol. 122, September 2000, pp. 573-576, incorporated by reference herein. As disclosed therein, the track includes a pattern of non-uniform sectors that are detected by a set of sensors distributed along the track, each sensor representing one bit of the absolute position word. In order to achieve consistent transitions during motion, the arrangement needs to follow a Gray-type pattern, i.e., only one sensor can change state at a time.
A conventional resolver is a type of rotary electrical transformer. In a brushless configuration, a primary winding, fixed to the stator, is excited by a sinusoidal electric current, which induces current in the rotor regardless of its relative angular position. The current then flows through another winding in the rotor, in turn inducing current in a pair of secondary windings, which are configured at 90° from each other in the stator, to produce sine and cosine output signals. The relative magnitudes of the outputs are used to determine the angle of the rotor with respect to the stator. Conventional resolvers typically suffer from a limited resolution. This may be overcome with an increased number of poles. Increasing the number of poles results in higher complexity and may be difficult to adapt to applications with a broad range of operating temperatures.
Conventional linear and rotary variable differential transformers typically utilize three coils (a central primary coil and two outer secondary coils) and a ferromagnetic core attached to the object the position of which is to be measured. Alternating current is driven through the primary coil, causing a voltage dependent on the position of the core to be induced in each secondary coil. If the secondary coils are connected in reverse series, so that the resulting output voltage is the difference between the two secondary voltages, the amplitude of the output signal is proportional to the position of the core.
In a ratiometric arrangement, where the individual output signals from the two secondary windings are compared to their sum and by utilizing the sum of the signals as a reference input for feedback control of the AC excitation, the sensor offers an exceptional level of tolerance to temperature variations. See e.g., Ara K., “A Differential Transformer with Temperature-and Excitation-Independent Output”, IEEE Transactions on Instrumentation and Measurement, Vol. IM-21, No. 3, August 1972, pp. 249-255, incorporated by reference herein. If properly designed, the sensor may also be insensitive to external electromagnetic interferences. See e.g., Martino et al., “Design of a Linear Variable Differential Transformer with High Rejection to External Interfering Magnetic Field”, IEEE Transactions on Magnetics, Vol. 46, No. 2, February 2010, pp. 674-677, incorporated by reference herein. Taking advantage of Fe-rich amorphous wires for the core, a small exciting field and a small number of windings in the secondary coils may be necessary to obtain a large output signal. See e.g., Hristoforou et al. “Linear Variable Differential Transformer Sensor Using Fe-Rich Amorphous Wires as an Active Core”, Journal of Applied Physics, Vol. 87, No. 9, May 2000, incorporated by reference herein. One main drawback of LVDT's and RVDT's may be a limited range of motion.
A high-resolution multidimensional position encoder that simultaneously measures positions of an object in plane and senses distance of the object in the perpendicular direction was described in U.S. Pub. No. 2009/0224750, incorporated by reference herein. As disclosed therein, the encoder utilizes Hall-effect read-heads to sense a three-dimensional magnetic field produced by an array of permanent magnets that are embedded in a forcer of a planar electric motor.
Extending the same principles to a rotary maglev application, a high-resolution encoder for simultaneous measurements of an angular orientation and eccentricity of a motor rotor was developed. In this case, Hall-effect read-heads sense a two-dimensional magnetic field from the permanent-magnet rotor and also detect features on a secondary track to determine the absolute angle of the rotor. See e.g., U.S. Pub. No. 2009/024313, incorporated by reference herein. Since a separation barrier can be used to isolate the read-heads from the rotor, this approach is suitable for vacuum applications. However, the presence of the magnets in a potentially aggressive environment and a limited operating temperature range disqualify this technology from the extreme applications under consideration.
A concept of a high-resolution two-dimensional encoder that measures simultaneously linear position and gap of a passive magnetically levitated cart was described in U.S. Pub. No. 2009/0033316, incorporated by reference herein. The encoder is based on a series of variable differential transformers, which are located along the motion path of the cart to sense a single ferromagnetic element coupled to the cart. In some respects, this approach can be viewed as an inverted version of the present concept. However, it is considerably more complex due to the specific requirements of the target maglev application.
A prominent place among the technical challenges in the subject application belongs to environmental effects, as the proposed system for position sensing needs to operate in vacuum and under extreme temperatures.